When HTTP headers are placed into the WSGI environ, they are normalized by
converting to uppercase, converting all dashes to underscores, and prepending
HTTP_. For instance, a header X-Auth-User would become
HTTP_X_AUTH_USER in the WSGI environ (and thus also in Django’s
request.META dictionary).

Unfortunately, this means that the WSGI environ cannot distinguish between
headers containing dashes and headers containing underscores: X-Auth-User
and X-Auth_User both become HTTP_X_AUTH_USER. This means that if a
header is used in a security-sensitive way (for instance, passing
authentication information along from a front-end proxy), even if the proxy
carefully strips any incoming value for X-Auth-User, an attacker may be
able to provide an X-Auth_User header (with underscore) and bypass this
protection.

In order to prevent such attacks, both Nginx and Apache 2.4+ strip all headers
containing underscores from incoming requests by default. Django’s built-in
development server now does the same. Django’s development server is not
recommended for production use, but matching the behavior of common production
servers reduces the surface area for behavior changes during deployment.

Django relies on user input in some cases (e.g.
django.contrib.auth.views.login() and i18n)
to redirect the user to an “on success” URL. The security checks for these
redirects (namely django.utils.http.is_safe_url()) didn’t strip leading
whitespace on the tested URL and as such considered URLs like
\njavascript:... safe. If a developer relied on is_safe_url() to
provide safe redirect targets and put such a URL into a link, they could suffer
from a XSS attack. This bug doesn’t affect Django currently, since we only put
this URL into the Location response header and browsers seem to ignore
JavaScript there.

In older versions of Django, the django.views.static.serve() view read
the files it served one line at a time. Therefore, a big file with no newlines
would result in memory usage equal to the size of that file. An attacker could
exploit this and launch a denial-of-service attack by simultaneously requesting
many large files. This view now reads the file in chunks to prevent large
memory usage.

Note, however, that this view has always carried a warning that it is not
hardened for production use and should be used only as a development aid. Now
may be a good time to audit your project and serve your files in production
using a real front-end web server if you are not doing so.

Given a form that uses ModelMultipleChoiceField and
show_hidden_initial=True (not a documented API), it was possible for a user
to cause an unreasonable number of SQL queries by submitting duplicate values
for the field’s data. The validation logic in ModelMultipleChoiceField now
deduplicates submitted values to address this issue.

The default iteration count for the PBKDF2 password hasher has been
increased by 25%. This part of the normal major release process was
inadvertently omitted in 1.7. This backwards compatible change will not
affect users who have subclassed
django.contrib.auth.hashers.PBKDF2PasswordHasher to change the
default value.

Fixed a crash in the CSRF middleware when handling non-ASCII referer header
(#23815).

Fixed a crash in the django.contrib.auth.redirect_to_login view when
passing a reverse_lazy() result on Python 3 (#24097).